Crude oil and natural gas contain wet hydrogen sulfide. Such an environment is called a sour environment. Line pipes transport crude oil or natural gas produced from oil wells or gas wells. Therefore, the line pipes are used in sour environments. Line pipes used in such sour environments have a problem of hydrogen embrittlement attributable to hydrogen sulfide.
The hydrogen embrittlement includes sulfide stress cracking and hydrogen-induced cracking (hereinafter referred to as HIC). Sulfide stress cracking occurs in a steel product under static external stress. HIC occurs in a steel product with no external stress thereon. Line pipes are less subject to static external stress compared to oil well pipes. Thus, line pipes are especially required to have HIC resistance.
It is known that, in general, HIC occurs easily with an increase in steel strength.
Techniques for enhancing the HIC resistance of a steel product for a line pipe are proposed in Japanese Patent Application Publication No. 54-110119 (Patent Document 1), Japanese Patent Publication No. 58-18967 (Patent Document 2), Japanese Patent Application Publication No. 52-111815 (Patent Document 3), Japanese Patent Application Publication No. 61-60866 (Patent Document 4), Japanese Patent Application Publication No. 2004-176172 (Patent Document 5) and Japanese Patent Application Publication No. 2004-143593 (Patent Document 6).
The steel for a line pipe disclosed in Patent Document 1 contains Ca and Ce, and MnS in the steel is spheroidized. Patent Document 1 describes that the HIC resistance of the steel for a line pipe is thereby enhanced.
The steel for a line pipe disclosed in Patent Document 2 has a chemical composition containing Cu and Ni as essential elements and further satisfying Ca/S≤2.0. Patent Document 2 describes that the HIC resistance of the steel for a line pipe is thereby enhanced.
The steel product for a line pipe disclosed in Patent Document 3 has the reduced contents of elements that are easily segregated such as Mn, P and S and further contains alloying elements such as Cu, Ni, Cr and Mo. Patent Document 3 describes that entrance of hydrogen to the steel is thereby suppressed, enhancing the HIC resistance of the steel product for a line pipe.
The steel product for a line pipe disclosed in Patent Document 4 contains Ni, Cr and/or Mo. Patent Document 4 describes that entrance of hydrogen to the steel is thereby suppressed, enhancing the HIC resistance of the steel product for a line pipe.
The steels disclosed in Patent Documents 5 and 6 contain Mo and V as essential elements, and ferrite is precipitated on the grain boundaries of the quenched structures of bainite and martensite, suppressing crystal grain boundary embrittlement. Patent Documents 5 and 6 describe that a yield strength of 483 MPa or more and excellent HIC resistance can thereby be attained.